Homonojirimycin glycosides have been described in U.S. Pat. No. 4,634,765 as inhibitors of carbohydrate digestive enzymes and as antidiabetic compounds. The indicated compounds are prepared by the reaction of a protected glycosyl halide with an appropriately protected homonojirimycin compound. In the process described in the patent, the protected homonojirimycin compound is obtained by a cumbersome multi-step synthesis starting with the tetrabenzyl ether of D-glucopyranose. Thus, while the products in the patent would be available by the procedure described there, a method that would avoid the cumbersome synthesis would be attractive. Homonojirimycin itself was not used as an intermediate in the preparation of the protected homonojirimycin compound in U.S. Pat. No. 4,634,765 but it could be used in the overall synthesis if there was a procedure which would give the compound conveniently from available and inexpensive starting materials. Actually, such a procedure would have further value if it could also be used for the preparation of nojirimycin (a known glucosidase inhibitor) and desoxynojirimycin, with these indicated additional compounds obtained either specifically as intermediates or by appropriate modification of the procedure at some point. However, such a convenient procedure has not been available.
One attractive and available starting material for the synthesis of compounds of the type discussed above would be D-glucuronolactone and reports have appeared in the literature on the use of this material in stereospecific syntheses of polyhydroxylated cyclic amino acids and also the conversion of such an amino acid to desoxynojirimycin. Specifically, Bashyal et al., Tetrahedron, 43, 415 (1987) describes procedures whereby D-glucuronolactone is reacted with acetone to give the acetonide and then the free C-5 OH is converted to the corresponding azide. By proper choice of reactions, it is possible to obtain either of the stereoisomeric azides. Bashyal then describes the catalytic reduction of the azide to the corresponding amine with the reaction mixture being treating immediately with benzyl chloroformate so that the amine product of the reduction is actually isolated as the corresponding carbamate. The acetonide group is then cleaved with acid to give the corresponding dihydroxy compound which is then hydrogenated in acetic acid to give, by a series of reactions, a trihydroxypipecolic acid. Bashyal also describes the hydrolysis of the azido acetonide to remove the acetonide and give the corresponding dihydroxy compound, followed by catalytic hydrogenation in acetic acid to also give a trihydroxypipecolic acid.
Bayer German OLS 36 28 486 also includes a similar conversion of an azido acetonide to a trihydroxypipecolic acid and, while the Bayer procedure appears to consist of more individual reaction steps, there was no effort to isolate any compound until the final pipecolic acid was obtained. It is noted that Bayer also includes a description of the reduction of this acid with sodium borohydride and boron trifluoride to give desoxynojirimycin.
The synthesis of nojirimycin itself by an entirely different procedure using 5-amino-5-deoxy-1,2-O-isopropylidene-.alpha.-D-glucofuranose as an intermediate has been reported by Tsuda et al., Heterocycles, 27, 63 (1988). In that procedure, commercially available 1,2-isopropylidene-D-glucofuranose was used as the starting material. The regioselective oxidation of the C5-hydroxyl group in that compound gives the corresponding ketone which is then converted to the O-methyloxime. Reduction of the oxime then gives the amine referred to above. That amine is converted to nojirimycin via the bisulfite adduct by procedures which were previously reported. Although Tsuda indicates that his procedure would be a practical route to nojirimycin without chromatographical separation of the stereoisomers at any stage, nevertheless, it appears that chromatography is used to remove impurities and the conversion of the amine to nojirimycin actually gives a mixture of isomers and it is only because of the favorable crystallization of the nojirimycin bisulfite adduct that it is possible to obtain that material. In addition, although the second isomer remains in solution and does not affect the isolation of the nojirimycin adduct, the fact that it is formed in substantial amounts results in a reduction in the amount of nojirimycin that can be obtained.